An engine is used to mix a fuel with the air into a combustible mixture gas, and then guide the mixture gas into a cylinder, followed by sparking the mixture gas through a spark plug to cause the combustion of the mixture gas, so as to actuate a piston to reciprocally move in the cylinder. Then, the piston actuates a crankshaft through a connection rod, so that the crankshaft can be used to drive an external mechanism. Generally, the engine is applied to various industrial applications, such as vehicles (including automobiles or motorcycles) or generators, wherein the common engines can be classified into two-stroke type and four-stroke type. For a four-stroke type engine, one operational cycle thereof includes four strokes, as follows:
(1) Intake stroke: Referring now to FIG. 1A, in this stroke, an intake valve 101 of the four-stroke type engine is firstly opened, while an exhaust valve 102 thereof is closed. A piston 103 in a cylinder 100 is gradually shifted from an upper dead point U to a lower dead point L, so that an upper inner volume of the cylinder 100 above the piston 103 is increased. Thus, a predetermined vacuum condition is generated in the cylinder 100 for guiding the combustible mixture gas into the cylinder 100. After the piston 103 is shifted to the lower dead point L, the piston 103 has actuated a crankshaft 106 in a crankcase 105 to rotate a half circle (i.e. 180 degree) through a connection rod 104, so as to close the intake valve 101 and end the intake stroke.
(2) Compression stroke: Referring now to FIG. 1B, after finishing the intake stroke, the intake valve 101 and the exhaust 102 are simultaneously closed. The crankshaft 106 continues to rotate, so as to push the piston 103 to shift from the lower dead point L to the upper dead point U. Thus, the upper inner volume of the cylinder 100 above the piston 103 is decreased, so that the mixture gas in the cylinder 100 will be gradually compressed, while the temperature and the pressure of the mixture gas will be raised. When the crankshaft 106 further rotates a half circle to totally finish one circle (i.e. 360 degree or one revolution) and the piston 103 is shifted to the upper dead point U, the compression stroke is ended.
(3) Power stroke: Referring now to FIG. 1C, when the compression stroke is ended, a spark plug 107 is energized to spark the mixture gas for combustion. Because the combusted mixture gas rapidly explodes to cause the expansion thereof, the pressure and the temperature of the combusted mixture gas will be suddenly increased at very short time, so as to push the piston 103 to rapidly shift downward and cause the rotation of the crankshaft 106 through the connection rod 104 for actuating an external mechanism. During the power stroke, the piston 103 is shifted from the upper dead point U to the lower dead point L, and the crankshaft 106 further rotates a half circle to totally finish one and half circle (i.e. 540 degree). When the piston 103 is gradually shifted to the lower dead point L, the upper inner volume of the cylinder 100 above the piston 103 is increased, so that the pressure and the temperature of the combusted mixture gas will be gradually lowered,
(4) Exhaust stroke: Referring now to FIG. 1D, after the combusted mixture gas finishes its combustion to generate exhaust gas, the exhaust gas must be immediately exhausted out of the cylinder 100 for executing the next operational cycle. Thus, after the power stroke is finished, the exhaust valve 102 will be opened, while the piston 103 is gradually shifted upward to exhaust the exhaust gas through the exhaust valve 102. When the piston 103 is shifted to the upper dead point U, the exhaust valve 102 will be closed. At this time, the crankshaft 106 further rotates a half circle to totally finish two circles (i.e. 720 degree), that is also called an operational cycle.
As described above, the four-stroke type engine implements the foregoing four strokes to finish an operational cycle. In one operational cycle, the piston 103 is reciprocally shifted between the upper dead point U and the lower dead point L two times, while the crankshaft 106 is correspondingly rotated two circles. However, there are some technical problems existing in the traditional four-stroke type engine, as follows: In the exhaust stroke, some portion of the exhaust gas is generally remained in the cylinder 100 after finishing the previous operation cycle, so that the remained the exhaust gas affects the efficiency of the intake stroke and the power stroke of the next operation cycle. If the displacement of the piston 103 in the intake stroke, the compression stroke, the power stroke and the exhaust stroke is equal to each other, the ratio of the intake and exhaust volume can not be adjusted, so that the problem of the remained exhaust gas can not be solved. In addition, the crankshaft 106 must rotate one circle to finish the intake stroke and the compression stroke, but the crankshaft 106 can not output any effective power to drive the external mechanism at this time. In other words, the crankshaft 106 only can output effective power during the power stroke and the exhaust stroke. As a result, there is a problem of discontinuous acceleration between the first circle and the second circle of the crankshaft 106. Thus, the rotation speed of the crankshaft 106 is too fast, and components of the external mechanism may bear suddenly increased force, so as to substantially increase the problem of component abrasion and component life reduction. It causes that the traditional four-stroke type engine is not suitably applied to a high-speed rotation condition. To solve the foregoing problems, an engine must arrange two or more sets of four-stroke type engines side by side, in order to mutually compensate the power output vacancy with each other for increasing total power output. However, this arrangement increases the entire volume of the engine, so that it is disadvantageous to reduce the occupied space of the engine.
Therefore, it is necessary to provide an improved four-stroke type engine structure to solve the foregoing problems, as described above.